The present invention relates to an image forming apparatus used in, for example, a copy machine, a printer, a facsimile and the like.
In a full color image forming apparatus, an electric latent image is configured to be formed on peripheral surfaces of a plurality of photosensitive drums, which respectively correspond to each color of yellow (Y), magenta (M), cyan (C), and black (BK), by using a laser scanning unit, and toner images of each color are configured to be respectively generated by toner of each color.
The toner images of each color generated on the peripheral surfaces of the photosensitive drums are sequentially superposed and transferred to an intermediate transfer belt that endlessly travels, so that a color image is synthesized on the intermediate transfer belt. Next, the color image synthesized on the intermediate transfer belt is configured to be transferred to a sheet material including a paper and the like by using a transfer roller and then to be pressed and heated in a fixing device, resulting in a formation of a color image. Toner remaining on the surface of a photoreceptor without being transferred in a primary transfer unit is collected in a cleaning device of the photoreceptor and the collected toner is attached to the surface of a polishing roller, so that the photoreceptor is polished.
In many cases, in toner, silica for providing fluidity and metal particles, which are polishing particles for providing a polishing property, is used. Since the metal particles have lower electric resistance and also have a lower electric charge amount as compared with the silica and the like, they are easily detached from toner particles and easily remain on the surface of a photoreceptor without being transferred to the intermediate transfer belt in the primary transfer unit. Therefore, when a printing rate of an image becomes high, a ratio of the metal particles of the toner collected in the cleaning device becomes high.
Since an outer diameter of the metal particle is very small (about 1/100 of the toner particle), the metal particle passes through a cleaning blade and is attached to a charging device of a downstream side, so that the contamination of the charging device is promoted.
A tandem type color machine has a plurality of photosensitive units of yellow, cyan, magenta, black and the like, but since printing rates of each color are largely different from one another in many cases, a difference occurs in rates of metal particles of toner collected in cleaning devices of each image forming station (hereinafter, simply referred to a “station”), so that a difference easily occurs in contamination levels of charging devices.
On the other hand, when the ratio of the metal particles of the collected toner is low, dielectric breakdown of a photoreceptor easily occurs. That is, since the metal particles of the collected toner release an electric charge, when the ratio of the metal particles of the collected toner is low, the toner is excessively electrified by friction in the cleaning device. Furthermore, when the toner is peeled from the photoreceptor by the cleaning blade, the toner is discharged to the photoreceptor, so that the dielectric breakdown of the photoreceptor easily occurs.
Consequently, it is preferable that the ratio of the metal particles of the collected toner is maintained in a predetermined range. Conventionally, for example, as disclosed in Patent Literature 1, bias of a cleaning brush of an intermediate transfer belt is turned OFF, and cleaning of the intermediate transfer belt is suspended. During one rotation of the intermediate transfer belt, toner remaining on the intermediate transfer belt is supplied to the photosensitive drum.